A constant velocity universal joint, which constitutes the running mechanism of an automobile, has an outer race member. In general, the outer race member is produced as follows. A workpiece made of carbon steel, which is a columnar member, is successively subjected to forward extrusion forming, upsetting forming, and backward extrusion forming, and finally the workpiece made of carbon steel is plastically deformed to have a shape of the outer race member. Before performing the forging processing as described above, the workpiece made of carbon steel may be heated to a predetermined temperature. That is, when the outer race member is produced, the warm forging or the hot forging may be performed.
The outer race member, which is formed and processed as described above, is cooled to room temperature, and then the outer race member is transported to some heat treatment equipment. In order to soften the outer race member and improve deformation ability in the heat treatment equipment, or in order to uniformize hardness, various types of heat treatments are carried out, including, for example, low temperature annealing, spheroidizing annealing, and normalizing.
Subsequently, shot blast treatment is performed to remove oxide scale or the like generated on the outer surface of the outer race member during the heat treatment. Further, lubricating chemical conversion coating, which is composed of zinc phosphate or the like, is formed on the outer surface. After that, ironing processing (sizing processing) is performed for the outer race member. Accordingly, the outer race member is finished to have a final dimension. The ironing processing is usually cold forging.
When the production process is carried out as described above, a large space is required in order to store the outer race member before applying the heat treatment. However, the reservation of the space for the purpose of the storage only is economically disadvantageous.
The heat treatment is performed, for example, while the outer race member, which is placed on a belt conveyer, is moved in a continuous type heating furnace. However, the period of time ranging from the entry of the outer race member into the continuous type heating furnace to the exit, in other words, the treatment time is long. For this reason, the production efficiency of the outer race member is low. Even when the furnace is changed to the batch type heating furnace, it is impossible to shorten the treatment time.
In any case, the heat treatment equipment for effecting the low temperature annealing, the spheroidizing annealing, and the normalizing is large. The investment in plant and equipment is high.
If the heat treatment is omitted in order to avoid the inconvenience as described above, then the outer race member is not softened, and also the hardness is not uniformized. Therefore, some breakage or crack may appear in the outer race member when the ironing processing is performed, and the dimension accuracy of teeth may be lowered when the teeth are provided on the shaft section of the outer race member.
In view of the above, it is desired to establish a heat treatment method which is completed in a short period of time and which can be carried out with simple equipment. For example, Japanese Laid-Open Patent Publication No. 5-302117 discloses a process in which only the tempering is performed while omitting the hardening. On the other hand, Japanese Laid-Open Patent Publication No. 5-255739 discloses a process in which a workpiece made of steel is subjected to plastic deformation processing at a processing rate of 45% to 65% at a temperature between the Ac1 and Ac3 points, followed by air cooling (natural cooling).
In the case of the heat treatment method described in Japanese Laid-Open Patent Publication No. 5-302117, the formed product, which is obtained after the forging processing, is subjected to natural cooling. Therefore, it is necessary to secure the space for storing the formed product. In other words, in the case of the heat treatment method described in Japanese Laid-Open Patent Publication No. 5-302117, it is impossible to reduce such storage space.
In Japanese Laid-Open Patent Publication No. 5-255739, the described processing method is warm forging. This method cannot be applied to a process in which cold forging or hot forging is performed.
For example, when the hot forging is performed, the phase in the metal microstructure causes transformation in the steel material which has the temperature lower than the temperature during the forging. Therefore, the metal microstructure sometimes becomes nonuniform. In this case, various characteristics of the steel material differ depending on the portions thereof. Therefore, a heat treatment method, which uniformizes the metal microstructure as homogeneously as possible, is desired. However, a heat treatment method has not been known, which makes it possible to uniformize the metal microstructure as homogeneously as possible and which is excellent in efficiency.